Mark D. Rotter

Measuring the stimulated-emission cross-section: a case study in Nd:GGG

The stimulated-emission cross-section is a fundamental optical parameter that is used to characterize laser-active media. Oftentimes, one has incomplete information as regards to the spectroscopy or doping concentration of the laser-active species. In this paper, we present a method for the measurement of the stimulated-emission cross-section which is useful if either the concentration of laser-active species or detailed spectroscopic information is not known a priori, or as a means of comparing the results of several methods of cross-section determination. The method requires only a few simple measurements and is amenable to media in bulk form.

Interaction of a high-power laser beam with metal sheets

Experiments with a high-power laser beam directed onto thin aluminum sheets, with a large spot size, demonstrate that airflow produces a strong enhancement of the interaction. The enhancement is explained in terms of aerodynamic effects. As laser heating softens the material, the airflow-induced pressure difference between front and rear faces causes the metal to bulge into the beam. The resulting shear stresses rupture the material and remove it at temperatures well below the melting point. The material heating is shown to conform to an elementary model. We present an analytic model of elastic bulging. Scaling with respect to spot size, wind speed, and material parameters is determined.

Temperature-dependent 780-nm laser absorption by engineering grade aluminum, titanium, and steel alloy surfaces

Abstract. The modeling of laser interaction with metals for various applications requires a knowledge of absorption coefficients for real, commercially available materials with engineering grade (unpolished, oxidized) surfaces. However, most currently available absorptivity data pertain to pure metals with polished surfaces or vacuum-deposited thin films in controlled atmospheres. A simple laboratory setup is developed for the direct calorimetric absorptivity measurements using a diode-array laser emitting at 780 nm. A scheme eliminating the effect of convective and radiative losses is implemented. The obtained absorptivity results differ considerably from existing data for polished pure metals and are essential for the development of predictive laser-material interaction models.

Evolution of a solid state laser

Lawrence Livermore National Laboratory (LLNL) has been developing compact solid state lasers since the 1990s. One of the first lasers to be developed utilized flashlamp pumped architecture and neodymium glass as the laser gain media. In the early 2000s, a diode pumped version of the original flashlamp pumped laser was designed and built, responding to the requirements that a laser system for the military be compact in both size and weight while creating significant power (~100 kW) for the missions envisioned. This paper will discuss the evolution of solid state lasers at LLNL and provide a glimpse into the types of capabilities that could be achieved in the near future.

Technical challenges for the future of high energy lasers

The Solid-State, Heat-Capacity Laser (SSHCL) program at Lawrence Livermore National Laboratory is a multi-generation laser development effort scalable to the megawatt power levels with current performance approaching 100 kilowatts. This program is one of many designed to harness the power of lasers for use as directed energy weapons. There are many hurdles common to all of these programs that must be overcome to make the technology viable. There will be a in-depth discussion of the general issues facing state-of-the-art high energy lasers and paths to their resolution. Despite the relative simplicity of the SSHCL design, many challenges have been uncovered in the implementation of this particular system. An overview of these and their resolution are discussed. The overall system design of the SSHCL, technological strengths and weaknesses, and most recent experimental results will be presented.

Development of a Process Model for CO(2) Laser Mitigation of Damage Growth in Fused Silica

A numerical model of CO2 laser mitigation of damage growth in fused silica has been constructed that accounts for laser energy absorption, heat conduction, radiation transport, evaporation of fused silica and thermally induced stresses. This model will be used to understand scaling issues and effects of pulse and beam shapes on material removal, temperatures reached and stresses generated. Initial calculations show good agreement of simulated and measured material removal. The model has also been applied to LG-770 glass as a prototype red blocker material.

The Use of Large Transparent Ceramics in a High Powered, Diode Pumped Solid-State Laser

The advent of large transparent ceramics is one of the key enabling technological advances that have shown that the development of very high average power compact solid-state lasers is achievable.

The Solid-State Heat-Capacity Laser

Heat-capacity operation of a laser is a novel method by which high average powers can be generated. In this paper, we present the principles behind heat-capacity operation, in addition to describing the results of recent experiments.

Pump-induced wavefront distortion in prototypical NIF/LMJ amplifiers: modeling and comparison with experiments

In large-aperture laser amplifiers such as those envisioned for the NIF and LMJ lasers, the geometry is such that the front and back faces of the laser slab are heated unevenly by the pump process. This uneven heating result in a mechanical deformation of the laser slab and consequent internal stresses. The deformation and stresses, along with a temperature-dependent refractive index variation, result in phase variations across the laser beam. These phase variations lead to beam steering which may affect frequency conversion as well as energy-on-target. We have developed a model which allows us to estimate the pump-induced wavefront distortion for a given amplifier configuration as well as the spatially-resolved depolarization. The model is compared with experiments taken in our amplifier development laboratory, AMPLAB.

Use of High-Power Diode Laser Arrays for Pre- and Post-Weld Heating during Friction Stir Welding of Steels

In this research a high-power diode laser array was used to preheat HY-80 steel to determine the efficacy of using a diode laser array for preheating prior to friction stir welding in order to reduce frictional forces thereby reducing tool wear and increasing welding speeds. Using instrumented plates the temperature profile using diode heating alone was determined in order to validate theoretical models. A high-power diode laser is proposed as a more cost effective and efficient means of preheating compared to other hybrid friction stir welding techniques. Parameters of the diode array were easily controllable and resulted in a preheated area that very closely matches the typical stir zone observed in friction stir welds. A proposed diode laser assisted friction stir welding system is presented, and it is hypothesized that the addition of diode laser preheating will improve tool life and/or increase welding speeds on steels.